Shavings have the oil within them and they are in a sealed plastic bag so cannot air out at all. Those oils put off a vapor that is bad for our birds to breath. The birds are right down in them with their faces and scratching and breathing it in.
With cedar boards they have been out to the air and dried out.
My coop is cedar and does not have a cedar odor at all. Has not bothered my birds and looks great !!!!!
I didn't put my birds in it for 2 weeks of hot weather to make sure there wasn't a problem. i used cedar boards meant for fencing.
I did not make my perches or nesting boxes out of cedar. I used pine.